Download CR75th Anniversary Commentary

CR 75th Anniversary Commentary
Special Lecture
Jacob, Monod, the Lac Operon, and the PaJaMa
Experiment—Gene Expression Circuitry Changing
the Face of Cancer Research
Stephen B. Baylin
See related article by Pitot and Heidelberger, Cancer Res 1963;23:
1694–700.
It is a virtually universal rule in science that if we step back to
reflect upon a field currently viewed as extremely dynamic and
novel, we find ourselves standing on the shoulders of those whose
seminal observations gave birth to it far earlier. For those of us
working in the fields of signal transduction and epigenetics within
the cancer research arena, this is absolutely the case when we
consider the brilliant realizations of Jacob and Monod that
regulatory networks control gene expression in bacteria (1–5).
Their recognition that expression of a single gene can be repressed
by another gene for response to regulatory cues from the environment ranks as one of the top, and now most heavily explored,
areas of biology in general and cancer biology. A review in Cancer
Research in 1961 by Pitot and Heidelberger not only pays tribute to
this pivotal work of Jacob and Monod but with the prescient
intent of predicting how the concepts might be woven into our
understanding of carcinogenesis (6). To say their predictions were
accurate would be an understatement, as is readily apparent from
today's marriage between the exploration of regulation of gene
expression and our current efforts to dissect basic mechanisms
underlying the origins, initiation, and progression of cancer. It
becomes evident as well that the studies of Jacob and Monod,
and their implications as visualized by Pitot and Heidelberger,
helped usher in a biology that underpins our current quest to
evolve new strategies for improving the management of cancer.
Hence the selection of the review by Pitot and Heidelberger for
inclusion in the current celebration of 75 years of publishing in
Cancer Research.
The revelations provided by Jacob and Monod started, as do
many great stories in science, with a series of epiphanies by the
younger investigator, Jacob, which he brought to conversations
with the more established scientist, Monod. They followed their
eventual joint excitement over the possibilities raised with a
series of experiments, conducted during 1958 through 1961 at
the Pasteur Institute in Paris. These resulted in their outlining a
model for gene regulation, which survives as a core paradigm
today. Their observations established the principle that to properly regulate response of an organism to changing environmental conditions, in specific bacteria for their experiments, a gene
circuitry exists wherein one gene product regulates control of
another gene. The result is a change in cellular phenotype for
cellular metabolism (2–5). Building on experiments for dem-
The Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins,
Baltimore, Maryland.
Corresponding Author: Stephen B. Baylin, Johns Hopkins University, CRB1, 1650
Orleans Street, Suite 541, Baltimore, MD 21287. Phone: 410-955-8506; Fax: 410614-9984; E-mail: [email protected]
doi: 10.1158/0008-5472.CAN-16-0865
2016 American Association for Cancer Research.
onstrating that lambda phage genes can be both induced and
repressed in bacteria, the investigators established that changes
in need for lactose utilization lead to negative regulation of
b-galactosidase (2–5). The circuitry for this switch formed what
is now famously known as the lac operon (1–5). The studies took
advantage of the mating system employed in bacteria, in which
the chromosomal material of the male is progressively injected
over time into the female, thus progressively carrying genetic
material with it. This allowed investigators to map male genes
by chromosome position as their entry facilitated gene expression events in the female. Toning down the sexual connotations for the literature, the seminal study of Jacob and Monod,
with participation of Arthur Pardee, was first published as a
preliminary report in 1958 where it was dubbed the "PaJaMa"
experiment (1, 3, 5). In this study, the investigators were able
to show that a gene lacl encoded a trans-acting repressor for the
lac gene. In this concept, the activity of the regulator gene is
induced when the repressor protein in the cytoplasm is
induced by a small molecular weight product generated by
the target enzyme. This circuitry paradigm contributes robustly
to mechanisms for pathway feedback inhibition.
The nature of the trans-acting molecules through which the
repressive process is mediated remained to be determined with
many discussions of whether direct DNA–DNA interactions,
RNA, proteins, etc., would play this role. These subsequent
discussions, held at the headiest of meetings attended by many
luminaries in the embryonic field of molecular biology, are
credited with leading to the discovery of mRNA as put forth in a
review by Alexander Gann (3). Herein is described a lunch in
Sydney Brenner's rooms in King's College on Good Friday, now
some 55 years ago, attended by Jacob, Brenner, Francis Crick,
Alan Garen, and others where "suddenly that afternoon it
became obvious—first to Brenner and Crick, and then to the
others present—that the PaJaMa experiment predicted an
unstable intermediate in gene expression," which was concluded to be RNA. This suggested to the attendees that the mediator
for the repressor action potentially "really did act at the genetic
level controlling production of the unstable mRNA. This discussion, continued that evening at a party at Crick's house, led
directly to the experiment by Brenner and Jacob, who, together
with Matt Meselson at Caltech that summer, demonstrated the
existence of mRNA. Separately, Jim Watson, Wally Gilbert, and
Francois Gros arrived at a similar result through different means
at Harvard" (3). The years to come in our current age of biology
have revealed that all of the hypotheses derived from the first
findings of Jacob and Monod were relevant and presaged the
findings of how many different ways such transacting events
can be molecularly mediated.
In decades following the above observations, the paradigm
of the lac operon and its constituent repressor binding to an
operator and inducer ushered in an era, ever growing today, for
our understanding of cellular control through signal
2060 Cancer Res; 76(8) April 15, 2016
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Lac Operon, Signal Transduction, Epigenetics, Carcinogenesis
transduction circuitry and the concepts embodied for heritability of resultant gene expression changes established by
epigenetic mechanisms (2–4, 7). It has been justifiably stated
that "few proteins have had such a strong impact on a field
as the lac repressor has had in Molecular Biology" (2). It is
hard to imagine, looking back, the degree to which the work of
Jacob and Monod would become a knowledge base to build
upon in elucidating the vast series of mechanisms used by cells
to interpret environmental cues in processes ranging from
development to those of adult cell renewing systems challenged
by a myriad of normal and abnormal stimuli. A staggering
portfolio of cellular machinery to implement these processes
continues to unravel in what we now investigate every day as
activation of, and heritably transmitting of, information from
cell signaling pathways. These include switches in patterns of
gene expression and the cell nuclear events that fix these gene
events, including looping between DNA regions for control by
gene enhancers of promoters, the roles of noncoding RNAs
such as long-noncoding and miRNAs, and the roles of DNA
methylation, chromatin, and nucleosome positioning in heritably locking in gene expression changes, which can all contribute to creating new cellular phenotypes (8). Indeed, one
may view this as the expansion of, and definition of mechanisms for, the types of gene circuitry proposed and documented
by Jacob and Monod.
With the above background in mind, it is remarkable how
quickly, and with such prescience, Pitot and Heidelberger
brought forth the concepts outlined in their 1961 Cancer
Research review (6). They hypothesized components of the
systems outlined by Jacob and Monod could be transposed to
a concept of induced phenotypes that are heritably perpetuated and maintained for cellular responses to short, transient
exposure to carcinogens (6). They theorized that ongoing
experiments in the carcinogenesis field suggested these above
interactions might possibly allow engendering of a malignant
cell without necessitating participation of genetic (DNA)
changes such as gene mutations. Critical to this proposal, they
envisioned a potential state of "reversion" which might allow
for changing the malignant phenotype back to the nonmalignant state (6). These concepts are dear to the heart of researchers on the continuing quest to outline the precise roles for
epigenetic alterations in the initiation and progression of
cancer and the possibility that targeting such changes, and/or
what controls them, could provide for potent cancer management strategies (9, 10). They perceptively weave the concepts
of Jacob and Monod into a possible alternative to the then
prevailing doctrine that "cancer may result from a direct
interaction of carcinogen with genetic material"—a theory
they reasoned had developed by "acceptance by many as the
mechanism of carcinogenesis on the basis of theoretical simplicity rather than of scientific data." As an alternative, Pitot
and Heidelberger considered, and deeply modeled, how the
findings of Jacob and Monod might lead to the possibility that
"a cytoplasmic interaction of a carcinogen and a target protein
could lead to a permanently altered and stable metabolic
situation without the necessity of any direct interaction of the
carcinogen and genetic material" (6). A critical feature of their
hypothesis was that "under the proper circumstances and
before chromosomal alterations occurred, the process might
be reversed and lead to the production of a normal from a
tumor cell."
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In their proposal, via a series of presented complex models,
they proposed multiple scenarios and different variations of
biochemical and genetic themes that could mediate their proposed interactions, arriving at the following bottom line prediction—that a carcinogen can bind to and interfere with the
repressor of a growth process, thus effectively negating function
of the repressor through a process of "cytoplasmic inheritance."
Thus, this interference is not dependent on continued presence
of the carcinogen in daughter cells as they divide (6). Clearly, in
modern parlance, we visualize these dynamics as proceeding
through the cytoplasm to the nucleus via a series of signal
transduction events that subsequently get abnormally fixed by
epigenetic processes involving DNA methylation, chromatin,
and changes in nucleosome position (8–10). Clearly, this
suggests a profound role of epigenetic abnormalities early
during cancer initiation and this possibility is the subject of
many investigations today (9, 10). In this regard, Pitot and
Heidelberger wisely articulate several key rules, and cautions,
inherent to their proposed mechanisms and this wisdom
enriches their predictions as they are playing out today. First,
they stress that "it must be apparent to the reader that we are
here dealing only with the earliest changes in carcinogenesis.
Once the altered regulation is established (possibly within
minutes or hours), other effects appear, such as aneuploidy,
increased glycolysis, apparent multiple enzyme deletions, etc.,
which are probably secondary to the primary changes" (6).
Second, "it is not our intention to rule out or deny the
possibility that chemical carcinogenesis is a consequence of
the direct interaction of the compound with genetic material.
Rather, it is our purpose to call attention to alternative explanations, based upon current concepts of metabolic regulation
and control, that permit the perpetuation of metabolic changes
brought about by the temporary interaction of the carcinogen
and a cytoplasmic protein" (6). Finally, they conclude that "by
the application of these or similar theoretical models, it is
possible to reconcile the large body of sound experimental
data on chemical carcinogenesis with current concepts of metabolic regulation, and early cancer could be considered as a
phenotypic rather than a genotypic disease" (6).
In reviewing the work of Jacob and Monod, John Beckwith
(5) provides a wonderful sentiment that might serve also as a
coda to the ingenious joining by Pitot and Heidelberger of the
lac operon story with the field of human carcinogenesis—"new
theories that become successful paradigms for their field, in
their initial form at least, do not provide a correct explanation
for all of the phenomena that are considered important to that
field." And, yet as implied here, any initial flaws in such
theories do not prevent their never being separated from the
body of invaluable work they help to spawn. Our understanding today of gene transcription is driving virtually every aspect
of basic and translational tumor biology, again reminding us
of our ride on the shoulders of those coming before. The
publication by Pitot and Heidelberger is, then, emblematic
of why we are celebrating 75 years of publishing in Cancer
Research.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Received March 23, 2016; accepted March 23, 2016; published online April
15, 2016.
Cancer Res; 76(8) April 15, 2016
Downloaded from cancerres.aacrjournals.org on June 18, 2017. © 2016 American Association for Cancer Research.
2061
CR 75th Anniversary Commentary
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2062 Cancer Res; 76(8) April 15, 2016
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Cancer Research
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Jacob, Monod, the Lac Operon, and the PaJaMa Experiment−−Gene
Expression Circuitry Changing the Face of Cancer Research
Stephen B. Baylin
Cancer Res 2016;76:2060-2062.
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